Proteins belong to the family of tumor necrosis factor and receptor thereof (TNF/TNFR) play an important role in various complicated biological modulating systems such as, for example, cell proliferation and differentiation, cell viability and death, production of cellular hormones and activation of immune cells, and etc. Among the family, there are several members which are particularly involved in the transmission of apoptosis signals and the modulation of immune system. Most of the members classified in the TNF receptor superfamily contain a death domain and are capable of transferring death signals. These members include TFNR-1, CD95/Fas/APO-1, DR3/TRAMP/APO-3, DR4/TRAIL-R/APO-2, DR5/TRAIL-R and the like. The superfamily shares a common molecular structure. Namely, they all possess a region containing from 3 to 6 repeating cycteines in their extracellular domain and they have similar amino acid sequences. In addition, these death receptors are also characterized in a conserved death domain consisting of about 80 amino acid residuals at their carboxyl terminal (Yu K. Y., et al., 1991, J. Biol. Chem. 274 (20): 13733-13736). It is known now that such a signal sequence is required and crucial in the transmission of death signals. The death domain will activate a series of pro-apoptotic protease caspase, leading the cell to apoptosis due to disruption of chromosome DNAs (Sheikh, M. S. and Fornace, A. J. Jr., 2000, Leukemia 14: 1509-1513; Douglas R. Green, 1998, Nature, 396: 629-630).
Recently, Avi Ashkenazi et al. (Nature, 396: 699-703, 1998) have found a new receptor member, DcR3/TR6, by searching the EST database. It has been found that the messenger RNA (mRNA) of decoy receptor 3 (DcR3) are expressed especially in lung tissue, rectum adenocarcinoma and certain endothelial cell lines. The expression of DcR3 mRNA is also induced in PMA/inomycin-stimulated Jurkat cell line. DcR3 contains 4 regions rich in cystein and it is a soluble protein. It is also found that DcR3 binds with FasL/CD95L and thereby inhibits the cytotoxic effect modulated by LIGHT and FasL/CD95L (Yu K. Y., et al., supra.). It is known that LIGHT is a ligand of HVEM/TR2 and LT β R highly expressed on activated T-cells and macrophages and that it leads certain adenocarcinoma cell lines to apoptosis by the signal transmission of LT β R. In addition, in the immune responses, several aspects of apoptosis are performed by the FasL-Fas system. For example, the control of peripheral clonal deletion and clonal expansion, as well as the modulation of cytotoxic T-cell activity and the like are all co-modulated by Fas and its ligand FasL. The study of Robert M. Pitti, et al. (Nature, 396: 699-702, 1998) has shown that DcR3 competes with Fas for the binding of FasL to inhibit the death signal transmitted by FasL. It is therefore suggested that certain tumour cells may avoid the attack of immune system by expressing large amount of DcR3.
The gene of DcR3 is first isolated from cells of human lung carcinoma and colon carcinoma and it is shown to be expressed in the carcinoma tissue of the alimentary canal. Chang, B. et al. (PNAS, 97(3): 1230-1235, 2000) has produced antibodies with DcR3 fragments and used the antibodies as an assay of tissue immunostaining. However, in the above reference, only polyclonal antibodies against DcR3 are produced for immunostaining and only the expression amounts of mRNA are shown. Therefore, it is not an ideal assay with respect to the specificity of the antibodies, as well as to the time and cost of detection. Moreover, the above reference fails to specifically indicate whether DcR3 exists in serum. It fails to disclose methods applicable to the clinical diagnosis of diseases either.
For the detection of diseases, especially of diseases related to cancers, there is a need for a fast, efficient and accurate method of detection to easily screen patients at early stages of cancers so that they can be subjected to more detailed examinations or further treatments at such early stages. In addition, with respect to high-risk groups having family histories of certain diseases and patients recovered from cancers, an easy, convenient, fast and accurate detection method can efficiently trace certain diseases on a regular base, so as to achieve early treatment with early detection. Enzyme-linked immunosorbent assay (ELISA) has been broadly applied in the detection of various diseases. The accuracy of the assay correlates closely with the antibody developed. Therefore, there is a pressing need for the search of a index protein capable of detecting multiple cancers and for the development of relevant detection kits therewith.